This invention relates generally to TVRO receivers for the reception of a wide range of satellite TV signals and, more particularly, to an automatic polarization adjustment system for automatically aligning an earth station antenna with the plane of polarization of the particular channel to which the TVRO receiver is tuned at any given time.
In a TVRO system, the satellite signals are received by an antenna (usually a paraboloidal dish) and converted to a lower "1st IF" frequency at the antenna site. This conversion may be effected by a down converter, which converts only a single channel to the 1st IF frequency, or a block converter, which converts all channels of a common polarity to a 1st IF block of frequencies ranging from 950 to 1450 MHz. This entire block of frequencies is then fed via coaxial cable to the receiver, which selects a particular channel for viewing and/or listening. In the receiver, the 1st IF signals are converted to a 2nd IF frequency range which traditionally has been centered at 70 MHz in most TVRO systems.
The signals transmitted by a satellite have a designated polarization determined by the mode of excitation at the transmitting antenna prior to propagation through space. For efficient reception, and maximum video and/or audio quality, the receiving antenna subsystem of the TVRO must be properly aligned with the polarization of the received signals.
To minimize interference among signals from satellite transponders using adjacent frequency bands, the signals from transponders using alternate 20-MHz frequency bands have a first polarization, e.g., horizontal, while the transponders using the intervening frequency bands have a polarization orthogonal to the first, e.g., vertical. The frequency-modulated signals transmitted in adjacent channels almost always overlap each other, and thus it is important to have the receiving antenna precisely aligned with the polarization of the desired channel in order to avoid interference from adjacent channels. The signals from local terrestrial microwave links also overlap the frequency bands of the satellite channels, and thus precise polarization alignment of the selected signals and the receiving antenna can also help reduce terrestrial interference (commonly referred to as "TI").
Furthermore, proper orientation of an earth station antenna for reception of polarized signals from one satellite does not mean that the same orientation will provide optimum reception from other satellites. There are numerous satellites in geostationary orbit today, and these satellites are azimuthally spaced from one another to avoid interference. Thus, as an earth station antenna is swept across the array of geostationary satellites, the polarization planes of the different satellites are slightly skewed relative to each other due to the azimuthal spacing of the satellites. As a result, the optimum plane of polarization of the earth-based antenna varies from satellite to satellite.
To properly align the antenna subsystem and the polarization plane of the incoming signals from any given satellite transponder, TVRO systems normally include polarizers which can adjust the relative alignment of the polarization of the incoming signals and the orientation of the antenna. One type of polarizer mechanically rotates the small probe that is included in the feedhorn of most earth station antennas, by means of a small servomotor which is powered by either the indoor receiver or an antenna positioner. A second type of polarizer adjusts the polarization of the incoming signal electronically, by changing the voltage applied to a coil wound around an electromagnetic ferrite core located at the throat of the feedhorn.